Process for coloring diamonds



abandoned;

United States Patent the'United States of'America as represented by' the United .States Atomic 'Ener'gy Commission No Drawing. :Continuation of application Ser- No, 340,137, Mar. 3.1953. This applichtioriApn'ZB, 195s, Ser.No.732 ,71

- z'claima' cran -isaj This invention relates to diamonds and .is concerned with aprocess for colouring diamonds. This application is a continuation of m copending application, Serial No. 340,137; filed March'3, 1953, now

. The majorityof diamonds, in their natural state, have at least'some' colour, while in many it' is quite marked. Apart from grey stone's (which arejmost common and for which the colour is, attributed to small inclusions near the surface), 1y' 11ow,.o1ive green or yellow green, and b w n'seeni to. be the most common; The orig'i'riof these eolours is not'clean] It is 'not even clear'whetherthe .coloufrisgenerally confinedjto the surfaces,',or distributed through the body of the stones. 1 Certainly, in some cases (yellowsgand ly'ellow greens); theeolohr may become paler on polishing and can often be removed by grinding and therefore appears to be connected with the surface. In such cases the colour-has been attributed to the effect of naturalra'dioactivityi There is"'littleevidence for the colouring of diamonds by, impurities.

- 'Ihechange in colour of diamond induced by radium 2,945,793 Patented July 19, .1 960 pale yellow, brown, or brown-green for the 10 hour irradiations' -to brown-red, brown, or brown-green for the 1200 hour irradiations. Using a simple spectrometer it was possible to'see the absorption at the blue end of the spectrum (which was strong after only 10 hours irradiation) decreasing as the temperature was raised. In addition to the colour change various absorption lines and bands appeared and disappeared.

The evidence suggests that diamonds are coloured only by particles of 's'uflicient energy-to displace atoms from their normal lattice sites intointerstitial sites, and that the colour centres are connected with these vacant lattice sites mg interstitial atoms. According to F. Seitz (Crystal Growth. A General Discussion of the' Faraday Society.' 1949, p. 271) about 25 ev. recoil energy is neededffor a carbon atom in diamond to be displaced, by'bombardment, from its normal lattice site to an interstitial site. I

Defects in a lattice structure caused by such displaceinent of atoms are known as Frenkel defects and will hereinafter be, referred to as such. They may occur singly in which case the'displacemen't from the vacant site isprobably small or in groups when the displacement maybe greater. i f

An object of thepr'esent invention is to provide a method of manufacturing blue and blue-green gemstones from substantially colourless and less valuable diamonds.

According to the present invention a process for colourns ill-the range blue to blue-green a substantially colorless diamond comprises the-step of subjecting said dia-.

.mondto radiations of an energy within a range selected radiation 'was'n'oticed at the beginning'of the century'by Sir William Crookesand others. .These early reports are not very consistent, probably because the irradiations were' weak and the colour ehanges vslight. However, it was established in 1923 by Lindand Bardwell (Amer. Min. 8, 171,1923) that diamonds are coloured green by the alpha particles from radium. Prolonged heating at 500 C. is said to have restored the original colour.

There have been several reports, in the last decade, of diamonds coloured by proton, deuteron, and alpha particle bombardment. Various colours from light green to dark brown, depending on the length of exposure and the intensity of the beam, were obtained. It seems probable that the brown colours were due to a combination of heating and bombardment.

Random irradiation of diamonds in an atomic pile shows that a green colour is induced which deepens as the irradiation proceeds until the diamonds actually become opaque. The time required to reach the opaque stage depends of course on the thickness of the diamond, but, for diamonds about 2 mm. thick, is about one week in a graphite moderated pile operated with a thermal neutron flux of 10 /cm. /-sec. Any fluorescence and phosphorescence the diamonds may have (excited by A 3650) is rapidly extinguished, being quite unnoticeable after a weeks irradiation. In general, induced radioactivity is weak and of short half lives of the order of ten hours. A week after irradiation it was not usually detectable with an ordinary Geiger counter.

Heating at temperatures up to about 600 C. reduced the intensity of the green colour, the higher temperatures 't'oproducemainly single-Frenkeldefectsin the diamond l3:lu:e vctztlouratitpnis believedto'be due to single Frenkel defects andjgreencolourationto groups of Frenkel defects. Also in, accordance iwithth'e. invention, a diamond colouredblue-green by-the above'process is reduced to a pnre blue colour by subsequent heat treatment. Electron bombardments are ableto introduce appreciable numbers of 'Fre nkel defects; for the maximum recoil energy which; may be given toanucleus in an elastic up to this figure being the more effective (a temperature collision is, very approximately:

Wm=2E(E.-}-2m c )/Mc (1) where E is the electron energy and m c and Mc are the rest energies of electron and nucleus respectively. Also,

the cross section for the recoil energy to lie between a value Wand Wm denoted by o' is approximately:

E (mev.) 3 1 0. 5 Wm (ev.) 2,150 860 32g" (barns) 1a 14 13 Thus, for an integrated flux of 2x10 electrons/cm. at least within the range 0.5 to 3 mev., the concentration of primary displaced atoms, at the surface upon which the beam was incident, would be about 3x10- This would fall off as the beam penetrated due to the decrease,

firstly, in flux and secondly, in energy. Due to the form a of the dependence of a on W one would expect that the majority of the primary displaced atoms would have received recoil energies very much less than Wm. The electron bombardment, therefore, is believed to produce mainly single Frenkel defects, although an appreciable inultiplication-leading 'to secondary, displaced atoms, might occur at electron energies above about?! mev.

On the basis that 25 ev. is needed for an atom to be displaced, the threshold recoil energy for colouring by electron bombardment would be 125 'kev. as calculated from expression (1) above. i i A practical energy range for electron bombardment to produce blue to blue-green colouration is from about 0.5 mev. to about 2 mev. Below 1 mev. pure blue is ob tained. At about 1 mev. and above the colour tends to blue-green which 'may be reduced to a pure blue by heat treatment for half an hour at a temperature in the region of 500 C.

In one practical example of the invention a number of small colourless diamonds were mounted in Woods metal on a water cooled pipe and subjected to bombard-' ment by electrons from a Van der Graaf machine at an energy of 1 mev. for about an hour giving an integrated electron flux of between 1 and 2 l0 electrons/cm). The diamonds assumed a bluecolour with a slight green tint, the penetration of the colour being about 0.5 mm. The diamonds were then heated at 500 C. for half an hour when they became a pure blue.

, In an atomic pile, neutrons of all energies up to 1 or 2 mev. are present and can give the atoms of a crystal lattice recoil energies up to:

where E is the neutron energy and M is the atomic mass in mass units. The average is just half this and the cross section for the'collisions is about 3 barns. In general, the recoiling atom will have suflicient energyto produce, in turn, further displacements. Thus, one fast neutron collision will give rise to many Frenkel defects conglomerated in a small volume of the lattice (this topic is discussed by Seitz in the paper referred to); It has been estimated, on the basis of Seitzs theory, that 0.001% of the atoms in diamondbecame Frenkel defects in a 10 hour irradiation in agraphite moderated pile operated with a thermal neutron flux of 10 sq./cm./ sec.

" A practical energy range for neutron bombardment to produce blue colouration or blue-green colouration capable of subsequent reduction to a pure blue colour by heat treatment is from 10030 1000 ev., a range which may be obtained just outside the uranium core in the moderator of a thermal neutron atomic pile. An integrated flux of from about 10 to 10 neutrons per cm. inversely. as thc size of the diamond is required. The contribution to thisefiect' bygamma radiation from an atomic pile would be negligible.

' The inventionmay also be carried out by means .of gamma rays. The elfect of such rays-is to produce electrons by Compton scatteringand the electrons in turn produce-Frenkel defects by elastic collisions with the nuclei. Gamma ray energies of about 1 mev. and above with an integrated flux of about 10 gamma rays/cm. are required. Several years irradiation would, however, be required to obtain this flux from known gamma ray sources such as Cobalt or a Van der Graaf machine with a suitable'targ'et. What is claimed is: I

1. The process of coloring substantially colorless diamonds' comprised of the steps of subjecting the colorless diamonds to irradiation by.electrons having an'energy of l mev. fora'pproximatel'y 1 hour givingan integrated electron fiux of between land 2X10 electrons/cm whereby the diamonds take on a blue color with a slight green tint, and then heating the irradiated diamonds at 500? C. for .half an hour whereby they become pure blue. 12. A process for coloring a substantially colorless diamond in-the range blue to blue-green comprising irradiating said colorless diamond with elcctrons having an energy within therang'e 0.5mev. to 2 mew'to'obtain an integrated electron flux of between 1 and2 10 'electrons/cmi .V References Ci ted in the'iile of this patent FOREIGN PATENTS 660,719" Great Britain Nov. 14,- 1951 V l OTHER REFERENCES Lind et al.:"A mer. Mining, vol. 8, p. 111 1923).

Pringsheim et al.: Z. Phys," vol; 133, p. 2 (1952). Seitz: Advances in Physics, vol. 1, p. 43 (1 952). 

1. THE PROCESS OF COLORING SUBSTANTIALLY COLORLESS DIAMONDS COMPRISED OF THE STEPS OF SUBJECTING THE COLORLESS DIAMONDS TO IRRADIATION BY ELECTRONS HAVING AN ENERGY OF 1 MEV. FOR APPROXIMATELY 1 HOUR HAVING AN INTEGRATED ELECTRON FLUX OF BETWEEN 1 AND 2X10**18 ELECTRONS/CM.2, WHEREBY THE DIAMONDS TAKE ON A BLUE COLOR WITH A SLIGHT GREEN TINT, AND THEN HEATING THE IRRADIATED DIAMONDS AT 500*C. FOR HALF AN HOUR WHEREBY THEY BECOME PURE BLUE. 